A fuel cell is a kind of power generator that extracts electrical energy by electrochemically oxidizing fuel such as hydrogen and methanol and, in recent years, has been receiving attention as a clean energy supply source. Among them, a polymer electrolyte fuel cell has a low standard operating temperature of around 100° C., and further has a high energy density. Therefore, it is expected to be widely applied as a relatively small-scale distributed-type power generation facility, or a power generator of a moving body such as an automobile or a ship. In addition, a polymer electrolyte fuel cell has been receiving attention also as a power supply for small-size mobile apparatus and portable apparatus, and is expected to be mounted in a cellular phone, a personal computer or the like, replacing secondary batteries such as a nickel hydrogen battery and a lithium ion battery.
A fuel cell is usually constituted of a cell provided as a unit in which electrodes of an anode and a cathode where the reaction responsible for power generation occurs and a polymer electrolyte membrane that becomes a proton conductor between the anode and the cathode constitute a membrane electrode assembly (hereinafter, sometimes abbreviated as “MEA”), and the MEA is sandwiched between separators. The polymer electrolyte membrane is constituted mainly of a polymer electrolyte material.
As a required characteristic of the polymer electrolyte membrane, proton conductivity under a low humidity condition can be mentioned. Cost reduction for practical applications to a fuel cell for an automobile, a fuel cell for domestic use or the like has been considered. By using a polymer electrolyte membrane having sufficient proton conductivity under a low humidity condition, the operation can be performed at a high temperature exceeding 80° C. under the low humidity condition of a relative humidity of 60% or less, and the water management system can be simplified.
Conventionally, as the polymer electrolyte membrane, Nafion (registered trademark, manufactured by E. I. du Pont de Nemours and Company) that is a perfluorosulfonic acid-based polymer has been widely used. Nafion (registered trademark) exhibits high proton conductivity under a low humidity condition through a proton conduction channel due to the cluster structure, but on the other hand is extremely expensive because of being produced through a multi-stage synthesis. Further, there has been a problem that fuel crossover is large due to the cluster structure described above. In addition, in a fuel cell operating condition, a wet-dry cycle is repeated, and a polymer electrolyte membrane repeats swelling and shrinkage. At that time, since the electrolyte membrane is constrained of a separator or the like, there has been a problem that the membrane is broken due to local stress concentration, and the mechanical strength or physical durability of the membrane are lost. Moreover, a problem that the electrolyte membrane has a low softening point and cannot be used at high temperature, a problem of waste disposal after use, and a problem that material recycling is difficult, have been pointed out.
To overcome such problems, a hydrocarbon-based polymer electrolyte membrane that is inexpensive and excellent in the membrane properties, and can replace Nafion (registered trademark) has been actively developed in recent years.
Among them, in particular, to achieve both of proton conductivity under a low humidity condition and mechanical durability of the electrolyte membrane, an attempt focused on a phase separation structure has been made. For example, in WO 2013/031675, a crystalline polyether ketone (PEK)-based polymer electrolyte membrane having a phase separation structure has been proposed. Further, for the purpose of suppressing the dimensional change accompanying the wet-dry cycle of an electrolyte membrane, an attempt focused on the combination of a reinforcing material with an electrolyte membrane has been made. For example, in JP 2013-62240 and JP 2010-232158, a composite polymer electrolyte membrane in which an electrolyte membrane is reinforced with a porous material or fiber nonwoven fabric made of polytetrafluoroethylene has been proposed.
However, we found that the following problems exist in the conventional techniques. The electrolyte membrane described in WO '675 has achieved the high mechanical strength due to a pseudo-crosslinking effect by strong crystallinity while maintaining high proton conductivity under a low humidity condition due to the phase separation structure. But even with this technique, the reduction effect of dimensional change in wet-dry cycle is not sufficient, and further improvement of the physical durability has been required.
In contrast, in JP '240 and JP '158, for the similar purpose, a hydrocarbon-based electrolyte is reinforced with a fluorine-containing porous membrane, but since the hydrophilization treatment for the fluorine-containing porous membrane is insufficient, affinity between the hydrocarbon-based electrolyte and the fluorine-containing porous material is poor, and a large number of voids are present in the obtained composite electrolyte membrane. Therefore, there has been a problem with fuel permeation and mechanical strength.
It could therefore be helpful to provide a polymer electrolyte membrane having excellent proton conductivity even under low humidity and low temperature conditions, further is excellent in mechanical strength and physical durability, and is capable of achieving high power, high energy density, and long-term durability when being used as a polymer electrolyte fuel cell; and a membrane electrode assembly and a polymer electrolyte fuel cell, using the polymer electrolyte membrane.